The present invention relates to the field of wireless telecommunication networks, in particular wireless local networks, or WLANs (Wireless Local Access Networks), compliant with the IEEE 802.11 family of standards.
Such networks are also called Wi-Fi networks. They are used in numerous applications to network stations (for example computers, personal digital assistants and peripheral devices).
The 802.11 standard defines in the document entitled “IEEE 802.11a-1999, IEEE 802.11b-1999, IEEE 802.11d-2001, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications” a method for controlling traffic in the wireless network. This method uses a Congestion Window (CW) system to control this traffic. According to this standard, in order to determine the instant at which to transmit a data packet, a station randomly draws a random number between 0 and CW-1, the value CW being an integer between two values CWmin and CWmax specified by the 802.11 standard.
This value CW is used as a countdown counter for the transmission of the packet, this counter being deferred if the station observes that another station is in the process of transmitting. Unfortunately, this congestion window system generates a large number of collisions on the wireless network, resulting in a significant loss of bandwidth from the point of view of the user.
A mechanism referred to as the “tournament method” can be used by the various stations to control the transmissions of packets and limit the collision rate. This tournament method is described in the document authored by Z. Abichar and M. Chang, and entitled “CONTI: Constant-Time Contention Resolution for WLAN Access”, IFIP Networking 2005.
The tournament method involves organizing a type of tournament between stations with at least one packet to transmit. A tournament is made up of a certain number of selection rounds, each executed over a time interval of predefined duration allowing each station to have time to hear the transmission of any other station. This duration can typically be equal to the duration of the time interval called “SlotTime” which according to the IEEE 802.11 standard is defined as an elementary interval in the procedure for contention resolution by congestion window. At the start of the tournament, all stations that have to transmit a packet are capable of being authorized to transmit this packet. At each selection round, one or more stations are capable of being removed from the list of stations authorized to transmit, according to whether an authorization or a prohibition to transmit is attributed to them during this selection round. At the end of the tournament, only the non-eliminated stations are authorized to transmit. If several stations remain in contention at the end of the tournament, they transmit at the same time thus causing a collision and therefore a disrupted reception, meaning that it is impossible to correctly receive the transmitted data packet. These stations will then have to participate in the next tournament to attempt to retransmit these packets.
The HiperLAN protocol is one of the first protocols incorporating the tournament feature.
For further information, the person skilled in the art can refer to the document authored by Philippe Jacquet, Pascale Minet, Paul Mühlethaler and Nicolas Rivierre, and entitled “Priority and Collision Detection with Active Signaling—The Channel Access Mechanism of HIPERLAN”, Wireless Personal Communications 4: 11-26, 1996.
Documents FR 2 893 206 and WO 2009/095628 explain, respectively, how to considerably improve the effectiveness of tournaments by making the probabilities of drawing dependent on the tournament history and how to ensure that the stations have restricted access times for the transfer of synchronous flows in wireless networks with a good quality of service.
However, the Applicant has observed that networks which implement a tournament method can suffer from a poor rate of bandwidth utilization, the duration of execution of the contention resolution protocol able to represent, for example in the example of a six-round tournament, about 25% of the time needed to transmit a data frame over an IEEE 802.11n network.